FILE - In this Jan 14, 2009 file photo, former Bay Area Rapid Transit police officer Johannes Mehserle, appears in the East Fork Justice Court in Minden, Nev. The defense has rested in the trial of Mehserle who is accused of murdering an unarmed black manon an Oakland train platform. Mehserle's defense called a forensic pathologist as its final witness before resting Tuesday, June 29, 2010.

Photo: Cathleen Allison, AP

FILE - In this Jan 14, 2009 file photo, former Bay Area Rapid...

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This image provided by the Los Angeles County Superior Court shows Oscar Grant who was shot by former San Francisco Bay Area Rapid Transit police officer, Johannes Mehserle on New Year's Day 2009. Mehserle testified Friday June 25, 2010 that he mistakenlypulled out his pistol instead of a stun gun when he shot and killed an unarmed black man who was lying face down on an Oakland train platform.

Photo: AP

This image provided by the Los Angeles County Superior Court shows...

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BART Police made available a Taser gun and holsters. Shared by fellow officers the night Oscar Grant was killed in a police involved shooting. Officers were expected to share Taser holsters, even though reconfiguring them to your preferred position required a wrench and a screwdriver. Attorneys for Officer Johannes Mehserle, charged with murdering of Grant, say he meant to pull his Taser rather than his gun Wednesday Feb. 3, 2010.

Johannes Mehserle made a tragic mistake. Or, if he didn't, he's telling a profound lie.

When jurors gather Tuesday to continue deliberations in the former BART police officer's murder trial, this one question will overshadow all others: Did Mehserle, 28, accidentally kill Oscar Grant while intending to shock him with a Taser during an arrest, or did he intentionally shoot the 22-year-old train rider in the back and later conclude that the consequences of admitting it were too fearsome?

The question is central to Grant's death at Fruitvale Station in Oakland on Jan. 1, 2009 - and what should be done about it. It could mean the difference between murder and manslaughter, or conviction and acquittal.

Prosecutor David Stein and defense attorney Michael Rains spent three weeks saying the correct answer is not just clear but unmistakable, so long as common sense is used.

Mehserle, a defense expert suggested, was like a driver who switches to an automatic transmission and then, in a pinch, jams his left foot into a clutch that's not there.

Not so, Stein said: "It's like getting into your car and reaching over to the glove box for the steering wheel."

Stein told jurors the shooting was intentional and that they should find Mehserle guilty of second-degree murder. Rains countered that only an acquittal - not murder nor manslaughter, whether voluntary or not - would serve justice in the case of a peace officer who slipped up under pressure.

Here are our summaries of the opposing arguments on this key question:

Prosecutor: Mehserle knew he had his gun

Tasers have been used hundreds of thousands of times by cops around the country. If Mehserle actually made the mistake he claims, he made history.

No officer has ever, while wearing a Taser in a holster positioned on his weak side, accidentally grabbed a gun from the strong-side hip and fired it.

Mehserle knew which weapon was which. Twice in the minutes before he shot Grant, he reached across his body with his right hand and pulled out his Taser. Grant even used his cell phone to snap a photo of Mehserle with the shock weapon out.

Mehserle's justification for deciding to use his Taser - that Grant, while lying on his chest, appeared to reach for a gun in a pants pocket - makes no sense. Had he believed that, Mehserle would have pinned the hand down or opted for his own pistol, not his Taser.

He wouldn't have gotten up from his knees and broken contact with Grant, giving Grant a chance to wheel around and fire.

When Mehserle says he made his fateful decision, he looked down to his right at the gun - not down to his left at the Taser. Then he held the gun out in front of him - in his line of vision - before he pulled the trigger.

Keeping his silence

Mehserle then looked alarmed, but that was because he had shot an unarmed man - one who by this time had both hands in the small of his back.

Had he really meant to use his Taser, he would have immediately told every person he could find that he had made such a mistake. He would have said it over and over.

But he didn't tell Grant, not even after Grant said, "You shot me." In fact, he handcuffed Grant after shooting him.

He didn't tell Officer Anthony Pirone, who had been helping him subdue Grant, instead explaining, "Tony, I thought he was going for a gun." He didn't tell at least three other officers he spoke to in the 10 minutes he remained on the platform.

Finally, after asking a close friend on the BART force to sit with him at police headquarters, he didn't tell him.

Defense: Mehserle thought he had his Taser

Why would Mehserle, a police officer with no reputation on the BART force for aggressiveness, intentionally shoot a man in the back in front of hundreds of witnesses?

Why would he want to kill a man he met just 2 1/2 minutes earlier, and with whom he had no quarrel?

What makes more sense is that Mehserle - who just started carrying a Taser three weeks before the shooting - made the same mistake that at least six other officers have made in the past decade, because Tasers began to take on shapes similar to guns.

The way BART deployed the Taser added to the confusion. Training lasted one day, with officers allowed to fire the weapon just once.

Varying holsters

Cutting costs, the transit agency didn't buy everyone a personal Taser and Taser holster, instead forcing officers to swap them between shifts. Mehserle couldn't practice drawing the weapon. And he carried it in whatever holster he could get his hands on - sometimes setting up for a weak-hand draw and other times for a strong-hand draw.

Mehserle's actions at the time of the shooting show a man trying to use a Taser, a weapon that would not have endangered a fellow officer and other train riders in the background with a possible ricochet.

Mehserle thought Grant might be reaching for a gun, so he shouted, "I'm going to Tase him." When he reached for his pistol, he struggled to yank it free from the holster. He only looked in that direction for a fraction of a second.

The retention lever that locks in the gun was plastic. Perhaps it was worn, and failed.

Under the stress of a chaotic platform, Mehserle turned to a motor skill - drawing his gun - that had become automatic after thousands of repetitions, rather than to a skill - drawing his Taser - that he had just picked up.

He rose to his feet, creating enough distance between him and Grant for the two Taser darts to spread out and incapacitate Grant. Then he fired just once, even though he was taught that multiple gunshots were often needed to stop a suspect.

When he looked at his right hand and saw the gun, he shouted, "Oh s-, I shot him."

Emotional wreck

Mehserle then jammed the gun back into its holster, even though he was taught to keep it up and look for other threats - to "scan and assess" - if he ever fired it.

He put his hands to his head before bending over in despair.

He cuffed Grant briefly so he could search him - standard protocol in such a situation.

From that point, he was an emotional wreck. Initially, he didn't tell anyone about the mistake because he was still figuring out what he had done. Later, he stayed silent because he was instructed to talk about the shooting only with his attorney.